1. Field of the Invention
The invention is directed to a wind tunnel for the testing of objects in subsonic, transsonic and supersonic speed ranges.
2. Description of Related Art
Aerodynamic simulation in wind tunnels is usually perturbed by so-called "wall interference". This wall interference is generated by the walls which surround the flow in a wind tunnel and thus cause a difference from the situation of an airplane in free flight (or any random craft in free surroundings). Due to the presence of such wind tunnel walls, it is either impossible to achieve the desired Mach number of the flow (ratio between speed and sonic speed), and/or the flow field around the test object is influenced unfavorably, i.e. the quality of the simulation is degraded.
Reduction of such perturbations due to the wind tunnel walls and, if possible, their theoretical description and computation, belong to the main concerns of aerodynamicists. Numerous approaches to solve the above problem have been found and tested, but as yet without fulfilling all demands of wind tunnel specialists.
For reducing the influences of the walls and for making these influences calculable, several approaches have been developed. One proposed solution consists in making the walls in the region of the test section porous, to perforate them or to provide them with longitudinal slots of different shapes. All of these three variants have the effect that parts of the main air-mass flow can evade into a kind of hollow cavity around the test section. Such "open" systems provide for considerable reduction of the perturbations caused by the walls; however, it has been found that this approach entails interference noises which degrade the quality of the main flow. Also, considerable difficulties are met when defining the boundary conditions for computation of the residual corrections.
According to a further currently used solution, the walls of a wind tunnel as a whole are arranged to be flexible for adaptation to the respective aerodynmic situation. Such walls are called "adaptive walls". In a rectangular test section, it is comparatively simple to achieve deformation of two mutually confronting walls for thus adapting them to the test object. Although this would allow for obtaining the correct Mach number and also for correction of perturbations of the main flow, it is not possible to reduce the residual span-width effects which occur on the wings of lifting models. Due to technical restrictions, simultaneous adaptation through deformation of four walls can hardly be realized in practice. Further, using adaptive walls requires a very high positional accuracy of these walls in case of high subsonic Mach numbers, so that the use of adaptive walls in large tunnels, especially under variable temperature conditions, becomes considerably more difficult because of mechanical complexity. By using a combination of flexible octogonal walls having boundary surfaces consisting of sliding slabs, the wall interferences are clearly reduced, but this advantage involves massive mechanical difficulties. Moreover, these flexible octogonal walls have not been tested yet on their fitness for the supersonic range.
From the periodical "Flugwiss. Weltraumforschung", No. 6, 1982, pp. 398-408, there is known a transsonic wind tunnel having longitudinal slots formed in its walls. For reduction of wall interferences, the slots on the inner sides of the walls have thin aluminium stripes bonded thereon, leaving a small gap having a width of 1.75 mm. In this known wind tunnel, individual adaptation to the test model is not possible. Furthermore, the thin alumnium stripes are not sufficiently stable in shape.
German Patent 35 24 784 C2 discloses a transsonic wind tunnel with perforated walls wherein plates are arranged behind the walls, the distance of the plates to the walls being variable for minimizing the wall interferences through adaptive adjustment of the plates.
All of the above outlined concepts suffer from the disadvantage that they necessitate extensive alterations in the wind tunnel walls so that the tunnels can be retrofitted only with difficulties or not at all.
It is an object of the invention to provide a wind tunnel wherein the flow conditions on the walls of the test section are influended in such a manner that the areodynmic situation generated in the central region of the test section is largely identical to free flight conditions (i.e. those prevailing without limiting walls).